drm/fb-cma-helper: Remove unused functions

The cma drivers use the drm_gem_framebuffer_helper functions now,
so remove drm_fb_cma_destroy, drm_fb_cma_create_handle,
drm_fb_cma_create_with_funcs, drm_fb_cma_create and
drm_fb_cma_prepare_fb.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1506255985-61113-11-git-send-email-noralf@tronnes.org